Posted by Mark (188.8.131.52) on 16:11:26 24/02/05
In Reply to:Re: adhesion posted by Gordon England
: : : Is it possible to measure thermal spray coating (WC-12Co) adhesion by using scratch method or this method is suitable only for very thin coatings
: : : Thank you
: : _________
: : I suppose you are right that scratch test is useful to measure the adhesion of thin films such as those by PVD, etc, and would not be useful on thick coatings.
: : As you may well know, during the scratch test, a stylus in constant contact under increasing normal load F(N) with the tested surface undergoes relative motion. The lateral driving force F(L) needed to maintain such a constant movement has to be increased correspondingly as the F(N) increases. A real-time record or F(L) as a fucntion of F(N) curve is taken.
: : Depending on the thickness of the film, the initial F(L)-F(N) curve is more or less linearas a result of the increasing contact area with the normal force until when a sudden fluctuation of the F(L) is observed - this marking the onset of the film de-hesion ( the loss of adhesion).
: : The reason the scratch test can successfully probe the coating to the de-hesion point is that the contact stress field in the subsurface (using the traditional Herzian elastic contact theory is a good estimate)can only reach/maximise at a limited depth - a depth good enough to destroy a thin film.
: : For a thick coating over 100-250 um say of a thermal spray coating, the shear stress transmitted to this depth would be virtually zero, thus such a coating can not be failed by adhesion under normal loading range of a typical scrach test machine.
: : Hope this would be helpful.
: : Hong Wei Wang
: : Greenhey Engineering Services, UK
: I agree, scratch tests are not suitable for coatings of this thickness. Other than performing tensile bond strength testing, bend tests can be used as a good qualitative indicator.
: Regards Gordon
For further information about the scratch tester take a look at http://www.pvd-coatings.co.uk/scratch-adhesion-tester.htm
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